Formation of an aluminide coating, incorporating a reactive element, on a metal substrate

ABSTRACT

The reactive element is introduced to the surface of the metal substrate in the form of an oxide powder and the aluminide-type coating is then formed.

BACKGROUND OF THE INVENTION

[0001] The invention relates to the formation on a metal substrate of aprotective coating of the aluminide type incorporating at least onereactive element.

[0002] The field of application of the invention is that of theproduction or repair of metal components which, because of their use athigh temperatures and in an oxidizing medium, must be provided with aprotective coating.

[0003] The invention is especially, but not exclusively, applicable togas turbine components, in particular to components of the hot parts ofturbojets.

[0004] To optimize their operation, it is endeavoured to make gasturbines, especially turbojets, operate at the highest possibletemperatures.

[0005] The components exposed to these temperatures are usually made ofa refractory metal alloy, or superalloy, based on nickel or cobalt.

[0006] In order to improve their high-temperature behaviour, inparticular their corrosion and oxidation resistance, it is well known toform a protective coating on the superalloy metal substrate.

[0007] Among the constituent materials of such a protective coating,aluminide-type coatings, which especially allow the development of aprotective alumina film on their surface, are commonly used.

[0008] Aluminization by cementation is the technique used most often toform aluminide-type coatings. This technique generally consists inplacing the metal substrate in a closed chamber containing a cementationagent and in raising the assembly to a temperature usually between900°C. and 1150° C.

[0009] The aluminide-type coatings can be used by themselves, or incombination with an external coating forming a thermal barrier, such asa ceramic coating. In the latter case, the aluminide-type coatingconstitutes a bond coat between the substrate and the external coating,attachment of the latter being favoured by the presence of the aluminafilm forming an adhesion layer.

[0010] To increase the lifetime of the alumina-film-generating aluminideand to limit its deterioration by spalling it is known to incorporateinto the aluminide-type coating at least one reactive element usuallychosen from the group consisting of zirconium, yttrium, hafnium and thelanthanides.

[0011] Such a reactive element reinforces the diffusion barrier functionwith respect to elements of the metal substrate which are liable toaffect the alumina film, and it therefore favours the integrity and thepersistence of the latter. The presence of the reactive element alsoresults in a reduction in the rate of oxidation of the metal substrateand prevents the segregation, which is highly undesirable, of sulphur atthe interface with a ceramic external coating.

[0012] Various processes have been proposed for forming analuminide-type coating incorporating a reactive element.

[0013] A first type of known process consists in alloying or combiningseparately the reactive element with one or more constituents of thecoating and in forming the latter by a process involving physicaldeposition on the metal substrate.

[0014] For example, reference may be made to the document U.S. Pat. No.4,055,705 which describes the formation of a bond coat by the plasmaspraying or sintering of NiCrAlY or depositing it using another physicaltechnique. Reference may also be made to the document FR 96/15257 whichdescribes the deposition, by electrophoresis, or in the form of a paintwith a thermally degradable or volatile binder, of an MCrAlY (M being Niand/or Co and/or Fe) alloy powder on a metal substrate. Electroplatingan alloy containing a metal of the platinum group is then carried outbefore heat treatment and possible aluminization. Reference may also bemade to the document U.S. Pat. No. 5,824,423 which, although envisagingthe initial deposition of a reactive element on a metal substrate byphysical vapour deposition followed by aluminization, preferablyindicates the formation of a bond coat by the plasma spraying of an MAlY(M being Ni and/or Co and/or Fe) pre-alloyed powder.

[0015] These types of known processes require a further step of addingthe reactive element to an alloy. This may require major investment.

[0016] Reference may also be made to the documents SU 1 527 320 and SU541 896 which describe the application to the surface of a metalsubstrate of a suspension containing aluminium and zirconium powders anda binder, such as a varnish in solution, in order to obtain a protectivecoating after drying and heat treatment.

[0017] However, the handling of elements such as zirconium in dividedform is particularly difficult because of the high risk of spontaneousreaction with the air.

[0018] A second type of known process consists in forming an aluminiumcoating incorporating a reactive element by chemical vapour deposition(CVD). Reference may be made to the document U.S. Pat. No. 5,503,874which describes the alternating deposition of an aluminium layer and ametal oxide layer, such as yttrium oxide, zirconium oxide, chromiumoxide or hafnium oxide, from organometallic precursors. A heat treatmentallows the oxide to be reduced by the aluminium. Reference may also bemade to the document U.S. Pat. No. 5,989,733 which describes theformation of a coating by the chemical vapour deposition of the elementsAl, Si, Hf and possibly Zr, or another reactive element, preceded orfollowed by the electroplating of Pt, in order to obtain a modifiednickel aluminide.

[0019] These types of known processes require the use of a chemicalvapour deposition plant, which is expensive both in terms of investmentand maintenance.

[0020] A third type of known process makes use of the aluminizationtechnique, but by modifying it with the incorporation of the reactiveelement into the cementation agent. Reference may be made to thedocument FR 2 511 396 which proposes the use of a cementation agentcontaining aluminium, an aluminium alloy, an activator salt and areactive element.

SUBJECT AND SUMMARY OF THE INVENTION

[0021] It is an object of the invention to provide a process allowing analuminide-type coating incorporating at least one reactive element to beformed on a metal substrate in a simple and inexpensive manner.

[0022] This object is achieved by the fact that, according to theinvention, the process comprises the steps which consist in:

[0023] introducing the said reactive element to the surface of the metalsubstrate in the form of a powder of the oxide of the reactive element;and

[0024] then forming the aluminide-type coating.

[0025] Introducing the reactive element in the form of a powder of theoxide of this element makes it possible to avoid difficulties inhandling a powder of the reactive element.

[0026] The reactive element may be introduced to the surface of themetal substrate by coating with a composition containing the powdermixed with a liquid, or by spraying such a composition, or by sprayingthe powder on the substrate so that it becomes encrusted in its surface,or else by electrophoresis.

[0027] The process according to the invention is noteworthy in that,despite introducing the reactive element in pulverulent form, analuminide-type coating is obtained whose microstructure andeffectiveness are completely comparable to those of the similar coatingsof the prior art, whereas the method of implementation of the processproves to be particularly advantageous.

[0028] This is because the process does not require expensive equipmentto be installed or maintained.

[0029] The reactive element is furthermore introduced as close aspossible to the metal substrate, thereby optimizing the efficiencybetween mass of reactive element involved and doping of the coating thusformed.

[0030] In addition, it is possible for the mass of reactive elementintroduced to be controlled precisely and over a very wide range.

[0031] Furthermore, the process allows the reactive element to beintroduced into localized regions of the surface of the substrate, forexample for the purpose of repairing a protective coating. This is notpossible with the processes of the prior art, in which the reactiveelement is deposited in the gas phase or incorporated in a cementationagent.

[0032] The aluminide-type coating may be formed by aluminization afterintroducing the reactive element to the surface of the substrate. Nomodification to the known aluminization processes, apart from possiblythe duration, is necessary. This constitutes yet another advantage ofthe process.

[0033] As a variant, the aluminide-type coating may be formed bydepositing the constituents of the coating after the reactive elementhas been introduced to the surface of the substrate, and heat treatmentin order to make the constituents react together.

[0034] Again as a variant, at least the aluminium is furthermoreintroduced to the surface of the metal substrate in the form of powderand then the aluminide-type coating is formed by heat treatment. Thereactive element and the aluminium may be introduced to the surface ofthe substrate by coating or spraying with a liquid compositioncomprising a powder of the reactive element in oxide form, an aluminiumpowder and a binder, the coating or spraying being advantageouslycarried out in superposed layers in order to achieve a thicknessaccording to that of the desired aluminide-type coating.

[0035] According to yet another variant of the process, at least onemetal chosen from the group consisting of platinum, palladium, rhodiumand ruthenium is furthermore deposited on the surface of the substrate.

[0036] The aluminide-type coating formed by the process according to theinvention may be used by itself, or as a thermal barrier sublayer, anexternal coating made of ceramic then being formed which anchors to analumina film generated at the interface between the aluminide-typecoating and the ceramic external coating.

[0037] The invention also relates to metal substrates, especially gasturbine components made of a superalloy, which are provided withaluminide-type coatings as obtained by the above process.

[0038] The invention will be more clearly understood from reading thedetailed description given below by way of indication, but implying nolimitation.

DETAILED DESCRIPTION OF METHODS OF IMPLEMENTATION

[0039] The process according to the invention is intended moreparticularly, but not solely, for the production of aluminide-typeprotective coatings on metal substrates made of a superalloy, especiallya superalloy based on nickel or cobalt, such as metal substrates of gasturbine components, particularly turbojet components.

[0040] According to one characteristic of the invention, at least onereactive element that has to be present in the aluminide-type coating isintroduced to the surface of the substrate, prior to the formation ofthe coating, in the form of a powder of an oxide of the reactiveelement.

[0041] The reactive element is preferably chosen from zirconium,yttrium, hafnium and the lanthanides.

[0042] Depositing these reactive elements in the form of an oxide powdermakes it possible to avoid difficulties in handling these elements whichreact on contact with the air.

[0043] Several simple techniques may be used to deposit the oxide powderon the surface of the substrate.

[0044] A first technique consists in preparing a composition containingthe powder and a liquid, and in coating the surface of the metalsubstrate, or a selected part of this surface, with this composition.The liquid used is, for example, a resin to which a solvent mayoptionally be added. This makes it possible, after the resin hasoptionally been cured, to fix the powder to the surface. The coatingprocess may be carried out very conventionally using a brush.

[0045] As a variant, such a composition containing the powder and aliquid may be sprayed onto the surface or onto a selected part thereof.

[0046] Another technique that can be used consists in spraying only thepowder onto the surface of the substrate, or onto a selected partthereof. The spraying is carried out by giving the powder particlessufficient energy for them to be able to become encrusted in the surfaceof the substrate.

[0047] Yet another technique consists in depositing the powder on thesurface of the substrate by electrophoresis. This is a technique wellknown per se, a brief description of which may be found in theabove-mentioned document FR 96 15257.

[0048] It should be noted that, before introducing the oxide powder tothe surface of the substrate, an optional initial step of the processmay consist in forming, on the surface of the substrate, a coating madeof a precious metal chosen from platinum, palladium, rhodium andruthenium. Such a metal coating may be formed, in a manner known per se,by sputtering or by electroplating, a diffusion heat treatment thenbeing often carried out. As a variant, such a coating with a metal fromthe platinum group could be formed after introducing the powder of theoxide of an active element to the surface of the substrate.

[0049] The next step of the process consists in forming thealuminide-type coating.

[0050] Advantageously, a conventional cementation aluminization processis employed.

[0051] Pack cementation, with contact between a cementation powder andthe substrate, consists in varying the latter in a powder containing (i)an aluminium alloy, generally a chromium-aluminium alloy, (ii) an inertconstituent, such as alumina, in order to prevent sintering, and (iii) ahalogen-containing activator (for example, NH₄Cl, NH₄F, AlF₃, NaF, NaCl,etc.) which makes it possible to transfer the metal to be depositedbetween the cementation agent and the substrate. The assembly is raisedto a temperature of, for example, between 900° C. and 1150° C. in afurnace.

[0052] The cementation may also be carried out without contact with thesubstrate, the cementation agent being provided elsewhere in thefurnace. In the latter case, the halogen-containing activator may beincorporated into the cementation agent or may be introduced separatelyinto the furnace.

[0053] During the aluminization, the oxide of the reactive element,introduced beforehand to the surface of the substrate, may be at leastpartially reduced. When the oxide is dispersed in a resin, the latter israpidly degraded by the halides formed by the activator element and bythe heat.

[0054] Thermochemical reactions take place between the halides, thecementation agent, the oxide of the reactive element and the metal alloyof the substrate which make it possible to form the aluminide coatingand to disperse the reactive element within the aluminide coatingformed. With a substrate made of a nickel-based superalloy, a nickelaluminide containing the reactive element is obtained.

[0055] Processes other than aluminization may be used to form thealuminide-type coating. For example, constituents of the desired coatingmay be deposited on the substrate by physical vapour depositionprocesses, such as sputtering or plasma spraying, or chemical vapourdeposition processes using gaseous precursors. These processes are knownper se. Reference may be made, for example, to the documents GB 2 005729, U.S. Pat. No. 5,741,604 and U.S. Pat. No. 5,494,704. Theconstituents may be deposited as superposed alternating layers. A heattreatment is used to obtain the desired aluminide with possiblereduction of the oxide introduced beforehand to the surface of thesubstrate and dispersion of the liberated reactive element within thecoating.

[0056] According to yet another variant, a hybrid coat, consisting of apowder of the oxide of the reactive element and aluminium powder isdeposited on the surface of the metal substrate. The coat may bedeposited by a coating or spraying process using a compositioncontaining the oxide powder, the aluminium powder and an inorganic ororganic binder, such as a resin optionally diluted in a solvent. Severalsuperposed layers are formed according to the thickness of the coatingto be produced. A heat treatment is then carried out at a temperature ofpreferably between 800° C. and 1100° C. in order to form an aluminide bydiffusion from the metal substrate and the dispersion of the reactiveelement within the coating.

[0057] The metal substrate may be used just with the aluminide coatingproviding protection against corrosion and oxidation at hightemperatures.

[0058] It is also possible to add an external coating made of ceramic,for example zirconia, yttrium oxide or yttriated zirconia. This externalcoating, obtained by a physical deposition process such as, for example,sputtering, thermal spraying or electron beam evaporation, constitutes athermal barrier. The function of the aluminide-type intermediate coatingis then especially to act as a bond coat allowing attachment of theceramic external coating via an alumina film formed on the surface ofthe bond coat.

[0059] Examples of methods of implementing the process will now bedescribed by way of indication, but implying no limitation.

EXAMPLE 1

[0060] A metal substrate made of a nickel-based superalloy was providedwith a coating made of a zirconium-doped nickel aluminide in thefollowing manner.

[0061] A zirconia powder having a mean particle size of 14 μm was mixedwith a liquid acrylate resin in an amount of 1 part by weight of powderper 8 parts by weight of resin. The mixture was applied to the substrateby coating it with a brush and then the resin was cured by exposure toUV.

[0062] A contactless cementation aluminization operation was thencarried out by placing the substrate in a furnace in the presence of acementation agent and an activator. The cementation agent was composedof 30 wt % aluminium and 70 wt % chromium and the activator used wasNH₄Cl. The aluminization was carried out at a temperature ofapproximately 1100° C. for a time of approximately 4 h 30 min. Theacrylate resin was rapidly degraded by the halides formed and by theheat, while the zirconia was reduced.

[0063] Thus, a substrate made of a nickel-based superalloy with a nickelaluminide coating containing 0.9 wt % zirconium was obtained.

EXAMPLE 2

[0064] A metal substrate made of a nickel-based superalloy was blastedwith a zirconia powder identical to that of Example 1. The blastingallowed zirconia particles to be deposited on and encrusted in thesurface of the substrate.

[0065] A contactless cementation aluminization operation was thencarried out as in Example 1. The nickel aluminide obtained had azirconium content of a few hundred ppm, with a fine dispersion ofalumina particles having a size of less than one micron.

EXAMPLE 3

[0066] A metal substrate made of a nickel-based superalloy was coatedwith several layers of aluminizing paint. This paint consisted of thedispersion, in an inorganic binder, of a mixture of zirconia powder,aluminium powder, and silicon powder in respective proportions by weightof 8%, 82% and 10%. The layers were formed by coating the paint and weredeposited in succession with intermediate drying in air supplementedwith an oven treatment at 90° C. for 30 min. The number of layers waschosen according to the thickness of the aluminide coating desired.

[0067] The metal substrate was then placed in a furnace in order for itto undergo a heat treatment at 1000° C. in an inert atmosphere (argon).A nickel aluminide coating was obtained by diffusion, in which zirconiumwas dispersed.

[0068] As already indicated, depositing an oxide of the reactive elementby a coating or spraying process is advantageous in that it makes itpossible to form this coat on only part of the surface of the metalsubstrate. The most exposed critical parts of the substrate, or thoseparts of the substrate which require repair to the aluminide-typecoating or to the optional external ceramic coating, may therefore bechosen.

[0069] Although in the above examples the deposition of a zirconiapowder was envisaged, the process may be implemented in a similar mannerusing an yttrium oxide powder, a hafnium oxide powder, a lanthanideoxide powder or a mixture of two or more of these powders.

1. Process for the formation, on a metal substrate, of an aluminide-typeprotective coating incorporating at least one reactive element,characterized in that it comprises the steps consisting in: introducingthe said reactive element to the surface of the metal substrate in theform of a powder of the oxide of the reactive element; and then formingthe aluminide-type coating.
 2. Process according to claim 1,characterized in that a composition containing the powder in a liquidmedium is introduced to the surface of the metal substrate by coating itwith the said composition.
 3. Process according to claim 1,characterized in that a composition containing the powder in a liquidmedium is introduced to the surface of the metal substrate by sprayingthe said composition onto this surface.
 4. Process according to claim 1,characterized in that powder is introduced to the surface of the metalsubstrate by spraying the said powder so that it becomes encrusted inthis surface.
 5. Process according to claim 1, characterized in thatpowder is introduced to the surface of the metal substrate byelectrophoresis.
 6. Process according to claim 1, characterized in thatthe aluminide-type coating is formed by aluminization.
 7. Processaccording to claim 1, characterized in that the aluminide-type coatingis formed by depositing the constituents of the coating after thereactive element has been introduced to the surface of the metalsubstrate and heat treatment in order to make the constituents reacttogether and to disperse the reactive element within the coating. 8.Process according to claim 1 to 5, characterized in that at leastaluminium in pulverulent form is furthermore introduced to the surfaceof the metal substrate and the aluminide-type coating is formed by heattreatment.
 9. Process according to claim 8, characterized in that atleast one reactive element and aluminium are introduced to the surfaceof the metal substrate using a liquid composition comprising a powder ofthe oxide of the reactive element, an aluminium powder and a binder. 10.Process according to claim 9, characterized in that the liquidcomposition is deposited on the surface of the metal substrate asseveral superposed layers in order to achieve a thickness according tothat of the desired aluminide-type coating.
 11. Process according toclaim 1, characterized in that at least one reactive element chosen fromthe group consisting of zirconium, yttrium, hafnium and the lanthanidesis introduced to the surface of the metal substrate.
 12. Processaccording to claim 1, characterized in that at least one metal chosenfrom the group consisting of platinum, palladium, rhodium and rutheniumis furthermore deposited on the surface of the substrate.
 13. Processaccording to claim 1, characterized in that an external coating made ofceramic is formed on top of the aluminide coating.
 14. Process accordingto claim 1, characterized in that the aluminide-type coating is formedon localized areas of the surface of a metal substrate for the purposeof repairing a protective coating on the substrate.
 15. Metal substrateprovided with a protective coating comprising an aluminide-type coatingincorporating at least one reactive element and formed on the surface ofthe substrate, characterized in that the aluminide coating is obtainedby the process of claim
 1. 16. Metal substrate according to claim 15,characterized in that the protective coating furthermore includes anexternal coating made of ceramic anchored to the aluminide-type coating.17. Metal substrate according to claim 15, characterized in that thealuminide-type coating further incorporates at least one metal chosenfrom the group consisting of platinum, palladium, rhodium and ruthenium.18. Metal substrate made of a superalloy according to claim 15,characterized in that it constitutes a gas turbine component.